Is Vex Receiver compatible with other RC gear ?

I'm thinking of getting a Vex Transmitter/Receiver from eBay because they're so cheap these days.
I had the Vex kit for one night, and then took it back when I realized the
only thing there was enough parts for was the one and only thing they showed how to build --- too much for the money at the time. Now they're cheap as can be.
From what I remember, though, the receiver had non-standard connectors and was designed to only plug into the main processor. Does that sound right ? I only want it for a cheapo RC transmitter/receiver that I will use very infrequently.
Thanks for any help . JCD
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The RF receiver unit (small yellow box) uses a yellow 4 wire cable with standard 4 pin module phone plugs on it. (the ones you find on handsets, not the six pin connectors used for phone lines).
I just replaced the cable that came with VEX with the handset cord from my desk phone and it didn't work. The phone receiver had the wires crossed (pin 1 to 4, 2 to 3, 3 to 2, and 4 to 1) where as the VEX code had them straight through (1 to 1, 2 to 2, etc). But the connectors are the standard 4 pin module connectors.
I haven't worked with normal R/C gear so I don't know for sure what type of protocols and cables are standard in the R/C world but I believe this unit is something special designed just for the VEX. I believe the signal from the RF module is some sort of standard serial protocol but I'm not sure about that. It might just be a fairly normal pulse width modulated R/C signal for all I know. The R/F module doesn't have separate power so it get's it's power from this same 4 wire cable. I assume two wires are power going to the RF module and 2 are a serial signal of some type coming back to the controller.
I believe the connectors for the servos are also non-standard though the signals I believe are standard. I think the connectors might be same as one of the two R/C servo standards but I believe they are wired backwards on the VEX - that is, the servo is the female and the controller is the male - I believe (but am not sure) that the standard in the RC world is the other way around. So, without building custom cables, you can't plug the VEX hardware (controller or servos) with normal R/C gear. But I think all it would take is a simple custom cable.
Don't normal R/C receivers include both the function of the R/F receiver and the decoder to send the separate signals to each servo? With the VEX hardware, the yellow box R/F module doesn't generate servo signals for each channel. You must use the processor module to do that for you (or something else to do the function for you). So you can't just buy the VEX RF module to act as a cheap R/C receiver. You would have to buy both the RF module and the processor module before you can directly drive servos.

I don't much about the RC world, but I hope that helps....
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Curt Welch wrote:

The signals used between the receiver and the individual servos is a simple 5 volt logic compatible PWM signal. The repeat rate of the signal is about 50 Hz or once every 20mS. The rep rate is not critical. The information is in the width of the positive going pulse. The center position for servos is a 1.50 mS wide pulse. Minimum and Maximum widths are about 1.0 - 2.0 mS, different manufacturers use slightly different limits. I have never seen less than 0.900 mS or more than 2.10 mS on a modern radio.

The standard RC servo cable is 3 wires, +Batt, ground and signal. Some of the monster servos are coming with separate power leads so that you don't have to pull all of the motor current through the receiver power distribution, but they are the exception.

Yes.
> signals for each channel. You must use the processor module to do > that for you (or something else to do the function for you).
If the data stream looks like a standard RC transmitter data stream, this is easy to do in a microcontroller with a couple of timers. There is probably no reason to assume that it does though.
Good Luck, Bob
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Hello Curt and thanks for the info. Also, thanks to Bob H. who responded to this.
I just took a quick look-see thru eBay and along with your comments managed to refresh my memory about RC gear enough to know that the Vex kit is not what I need. The RC receivers that I am used to allow the servos to plugged directly into them --- which is what I am looking for. I don't want the extra trouble of *requiring* a processor in addition to the receiver - I want to be able to substitute the R/C receiver *in place of* a processor, so the Vex setup is not what I need.
Anyway, thanks a ton for your help! It made my decision easier and faster. Thanks ! JCD
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You can take the crystal out of the VEX receiver and plug it into a standard Futaba receiver and it works fine. very inexpensive way to get a transmitter and crystals.
Andy

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Correct. There is a GND, +5V, and signal. The other wire is "tether" which is used to tell it it's operating from a tether.
The signal is a "standard" servo signal, it's just 6 of them in a row.
ttyl,
--buddy
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Ok, cool. I had forgotten about the fact you could tether the transmitter dirrectly to to the controller.
How does it know which channel is which? Is there some type of start marker in the signal or is it just the signal spacing where there's a longer delay between groups than between pulses in the group?
I've got the VEX hardware but I've never worked with standard R/C equipment and I don't know a lot about what is available in that market, so I too have some questions maybe someone can help me with...
Is this VEX format the standard for R/C gear as it's transmitted over the air? Is it FM or AM or something else?
Does the VEX transmitter use the same standard as all other R/C gear so you could use it to control normal R/C devices or use other standard R/C transmitters to control a VEX device?
And is there one standard in the R/C word for hobby stuff for what is transmitted over the air or are there multiple standards?
Anyone ever create a digital system for the hobby market instead of using these analog pulse width system or is all the R/C world just using the old pulse width system?
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Curt Welch wrote:

In standard hobby radios, it is just a long "sync period" between pulses. Each channel follows the sync period in sequence. The sequence is not standardized between manufacturers.
If you look at the composite signal on a scope it will look something like this: |<------------ 1 Frame ------------------------------------->| _ _ _ _ _ _ _ _________| |___| |_____| |_____| |____| |____| |_____| |_______________|
| T1 | T2 | T3 | T4 | T5 | Tn | Tsync
T1..Tn are the periods of each pulse that will route to the individual servos. N may be as small as 1 or as high as 10. The frame period is about 20mS. The difference between the sum of all of the channel times and the frame time is the sync time. On "computer" radios I have looked at, the frame time is constant with the differences going into the sync period, The analog radios used a constant period sync time and the frame time varied as the sum of all the channel periods + the sync time. The decoders are made with parallel in/out shift registers or Johnson counters (see CD4017). The pulse train shown clocks the shift register or counter, while an RC time constant watches for a long pulse to reset the counter or parallel load a 1000000 pattern into the shift register. Individual servos are connected to the output bits on the counter or shift register. This arrangement lest you drive an 8 channel receiever/decoder with a 4 channel transmitter and not have stuff get confused. What I have shown here is an approximation. The polarity of the signals varies with the manufacturers implementation, and it has been a couple of years since I looked at one and I may have swapped it. The basic content and format is right though.

Mostly one standard with the PWM/PPM gear. For FM gear, some vendors deviate + during a pulse and others deviate -, but the timing is pretty standard. The signal format is generally referred to as PPM, but it looks like PWM to me.

Yes, all off the major vendors have what is called PCM which is actually a digital code transmission. As far as I know, all of the PCM formats are manufacturer proprietary and not released. I have seen web sites where people have been reverse engineering the formats, but all are works in progress as I remember.
Good Luck, Bob
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BobH wrote:

I forgot to add that the new spread spectrum 2.4GHz radios are probably sending a digital code transmission. I would bet a beer that they are using the numerous 802.15.4 chips, but would enjoy sharing a beer with somebody who has been inside one and could tell me.
Bob
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Thanks Bob for all the info!
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Good discussion guys.
When I was looking at Vex, it was apparent that significant engineering had gone into the product to make it non standard and specific only to the Vex product line....meaning that you would have to buy anything more you might need from Vex...at very high prices.
I suspect that the transmitter is stock (Futaba?) and the receiver has been modified to make it Vex specfic.
Personally I would go with a standard transmitter and receiver set...life is too short to reverse engineer every product interface....especially one which is dependent on a surplus source that is rapidly drying up and from a manufacturer who is going out of their way to make your life difficult.
TMT
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I wonder how much of that approach was the VEX/FIRST guys and how much was Radio Shack? I suspect it was mostly Radio Shack (or the intent to try and sell it through a retail store like Radio Shack) but don't know any of the inside scope on the history.
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On Aug 6, 12:22 am, snipped-for-privacy@kcwc.com (Curt Welch) wrote:

I too have thought about who was responsible.
I know it did not come free...someone spent significant engineering time to make sure that almost everything in the system was unique to their criteria..to lock the user into the sole source pricing...and that cost money and time.
I do know that because of that shortsightedness I do not use Vex related items unless absolutely necessary...again life is too short to waste time dealing with hardware and interfaces designed to waste my time..and to nickel and dime my budget.
TMT
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bubbajoe had written this in response to http://www.roboticscommunity.com/robotics/Re-Is-Vex-Receiver-compatible-with-other-RC-gear-18475-.htm : the vex radio is basicly a futaba 6EXA from my examination of the radio all the internal boards the carry the same part numbers as a futaba 6EXA so I tested it with futaba 127 on 72mhz as well as 75mhz recievers and it works flawless and can also be converted to 2.4 ghz
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bubbajoe had written this in response to http://www.roboticscommunity.com/robotics/Re-Is-Vex-Receiver-compatible-with-other-RC-gear-18475-.htm : vex radios are a futaba radio a 6EXA from checking the interna boards the carry futaba part numbers and I have tested them with futsaba 127 recievers and they work fine
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